The present invention relates, in general, to control valve equipment and, more particularly, this invention relates to a control mode to operate a valve device for use in an, Antilock Brake (ABS) and/or Traction Control Systems (TCS), for example, of a pneumatically operated vehicle brake system.
Both pneumatic and hydraulic braking systems employ a fluid under pressure to apply a braking force to a wheel. Upon detecting an inappropriately low wheel speed during vehicle braking, ABS may be invoked and the fluid pressure reduced. An inappropriately high wheel speed indicative of loss of traction may activate a TCS to apply fluid pressure slowing the slipping wheel and transferring torque to another non-slipping wheel. Many known antilock or anti-skid devices operate by cyclically increasing and decreasing the braking force exerted on the wheels so that a slipping wheel having a tendency to lock is permitted to re-accelerate back to a speed corresponding to the speed of the vehicle. This is typically achieved by control valves alternately allowing fluid to flow out of and then into the brake cylinder, thus lowering and then raising the brake pressure and frequently implemented by two solenoid operated valves, one of which admits fluid to the braking device and the other of which releases fluid from the braking device. The first valve provides either a building up or holding of existing fluid pressure while the other provides a release of the pressure. One-solenoid direct or pilot controlled valve devices for pneumatically operated vehicle braking systems equipped with an antilocking device are known. Especially in the beginning of antilock systems development in middle 1970""s, solenoid controlled relay valves were used in connection with complicated internal mechanical logic. Such systems provided a build up and release, but failed to achieve an adequate holding of a specific fluid pressure. Not only the variability of the pressure modulation was limited also the valve device was costly. It would be highly desirable to achieve the hold, build up and release functions with a single relatively inexpensive valve.
Many vehicles employ a differential which provides an application of power to both driving wheels even though one of those driving wheels may be turning faster or moving further than the other as, for example, when the vehicle is turning. If one of the differential coupled wheels is allowed to turn freely as when the tire slips on an icy road surface, no driving power is supplied to the other wheel. In an attempt to obviate this total loss of traction problem, so-called limited slip or positive traction differentials provide a clutch mechanism within the differential housing which, under normal driving conditions, slip allow the outside wheel to turn faster than the inside wheel. Under poor traction conditions, the increased friction provided by the clutches increases the driving torque applied to the wheel with the better traction.
The advent of TCS provided an opportunity to eliminate the weight and cost of these limited slip differentials. In some systems, the addition of a single traction control valve in the brake circuit of the powered wheels allows the anti-skid system to detect when the rotational speed of one driven wheel substantially exceeds that of the other and, despite the absence of any driver commanded braking, provides a braking force to the freely rotating wheel transferring torque back to the wheel with the better traction. Thus, traction control operation is simply a form of anti-skid operation when braking is not being called for.
ABS consists of wheel speed sensors (WSS) an Electronic Control Unit (ECU) and Pressure Modulation Valves (PMV). TCS used the same components as ABS but has an additional valve to apply brake pressure to brake a spinning drive wheel. Each individually controlled wheel requires its own WSS and PMV all electrical connected to one ECU.
The individual wheel speed sensors (WSS) are located at or in the vehicle wheels to sense the instantaneous movement of individual wheels and send an electrical signal to the ECU. The ECU monitors the WSS signals and determines when ABS or TCS intervention is required and actuates the appropriate PMV to optimize the brake pressure. The PMV is controlled electrically by the ECU to decrease, hold or allow building up the applied full brake pressure to prevent excessive wheel slipping or spinning.
For a proper Function of ABS or TCS a three-point pressure control with pressure hold, decrease or built up is necessary. This may be achieved utilizing two solenoid actuated valves or a single valve with two solenoids. If one solenoid is energized by the ECU, the pressure goes in hold position, when both solenoids are energized, the pressure decreases and when no solenoid is energized the pressure builds up. With a different valve design the dual solenoids can be energized one at a time to hold or decrease the pressure.
U.S. Pat. No. 5,660,449 discloses a two valve ABS system where fluid pressure is controlled by a solenoid valve having a variable duty cycle which is changed at predetermined intervals to move the solenoid in a plurality of steps toward a closed position. The two valves provide the conventional ABS mode of operation. U.S. Pat. No. 5,815,362 employs pulse width modulation for controlling an infinitely variable solenoid operated brake cylinder to achieve a pressure regulated feedback. This system is not concerned with ABS nor with achieving a pressure hold or rebuild mode of operation.
Also pressure valves with one solenoid are known. In this case normally only a two-point pressure control is possible: decrease and build up. The function of ABS or TCS with only one-solenoid valves is more or less degraded. The lower cost of an ABS/TCS with one-solenoid valves as compared to a system with two-solenoid valves is highly desirable. It is also highly desirable to achieve the hold function in addition to the decrease and build up using a single one-solenoid valve for each wheel.
The present invention provides a special control mode to operate a one-solenoid valve that is well suited for use in an antilocking device of pneumatically operated vehicle brake systems. This special control mode is provided by an Electronic Control Unit and allows modulating brake pressure of one or more brake chambers for pressure release, pressure hold and build up. The three-point pressure behavior is the fundamental control mode of the brake chamber pressure in an ABS event and normally achievable only with use of two-solenoid valves. A one-solenoid valve when normally operated provides only a two way pressure behavior with pressure build up if not energized and pressure release if the solenoid is energized. The special control mode of a one-solenoid valve is active in the pressure hold mode and when pressure is built up more slowly than normal. When a pressure hold or a more slowly pressure build up is required the one-solenoid valve is pulsated and intermittently energized to provide small pressure build up and release steps. The frequency and variation of the energizing duty cycles is in consideration of the actual pressure level. Two different one-solenoid valve arrangements are shown for use, for example, in antilock systems of pneumatically operated vehicle brake systems. The special control mode of an one-solenoid valve can be used in other pressure control systems, when a pressure hold function or different pressure build up gradients are required.
In general and in one form of the invention, a process of operating a vehicle braking system in an antilock braking mode includes enabling a single-solenoid valve to release braking pressure and thereafter alternately enabling and disabling the single solenoid valve to maintain a hold of braking pressure at a substantially constant reduced pressure. The solenoid is subsequently alternately enabled and disabled at a reduced ratio of enabled duration to disabled duration to rebuild braking pressure. The ratio of the time during which the valve is enabled to the time during which the valve is disabled is determined, in part, by the time during which the single-solenoid valve was initially enabled to release braking pressure and increases as the time during which the single-solenoid valve was enabled to release braking pressure increases.
With another method the ratio of the alternately enabling and disabling of the single valve is dependent to the respective brake pressure level in the ABS event. The actual pressure level in an ABS event is calculated by the ECU with an ECU internal stored pressure calculation program. The ratio of de-energized to energized time of the one solenoid duty cycles is based on the calculated pressure level in the similar way as the dependence to the release time T1. A lower calculated pressure level is equal to a longer release time T1 and a higher-pressure level is equal to a shorter pressure level T1.
One of the primary objects of the present invention is to provide a three-point pressure behavior with a one-solenoid valve for use with an antilock or traction control system or an electronic brake system for use mainly in pneumatically operated vehicle brake systems. The uses in other compressed air operated systems are also conceivable. A special control mode provided by an ECU is described to get this advantageous three-point pressure behavior with the low cost one-solenoid valve.
Other objects of the present invention include the use of the special control mode in combination with a single solenoid valve, a pilot controlled one-solenoid interrupter valve and with a pilot controlled one-solenoid relay valve.